Various ways to weld clear thermoplastic sheets into packages are known. The packages are typically made of clear plastic sheets (e.g., blister packages) by using RF welding methods, which are limited to a small number of thermoplastic materials (in practice, mostly PVC-based), which have the requisite electrical properties, such as a relatively high dielectric loss that provides the ability to convert alternating electric fields into heat. The process is not applicable or have very limited applicability to the most common plastics, from polyolefines to PC to Polystyrene to Polycarbonate, which in turn severely limits the material selection available for blister packaging. With a current campaign to limit the use of PVC because of its environmental hazard—recycling process for PVC is hazardous to various degrees due to the material's inherent chlorine content, as well as the range of chemical additives used for increasing material stability and variety in usage properties, the ability to utilize a wide range of thermoplastics for manufacturing of blister packages offers significant environmental benefits.
Another problem of RF welding is inflexibility of the process, as the seam is limited by the shape and size of the electrodes—each package's shape would require the electrodes of a specific shape and size. Also, in practice the size of the package is limited by the required power capacity, as the RF is known by very high power requirements to achieve material melting. For example, for a perimeter seal of a 10 cm×15 cm medical bag, the power requirements range from 1.7 to 2.9 KW (Plastics and Composites Welding Handbook, Hanser Gardner Publications, Inc., 2003. p. 262).
Laser welding of plastics has established itself as a robust, flexible and precise welding process, which is increasingly used to join plastic parts. It enables highly efficient and flexible assembly processes from small-scale production of parts with complex and varying geometries to a high volume industrial manufacturing where it can be easily integrated into automation lines.
Laser welding uses a laser beam to melt the plastic in the joint area by delivering controlled amount of energy to a precise location. This is based on the ease of controlling the beam size and the range of methods available for precise positioning and moving the beam.
The process is based on the same basic requirements of material compatibility as other welding techniques, but is often found to be more forgiving of resin chemistry or melt temperature differences than most other plastic welding processes. Nearly all thermoplastics can be welded using a proper laser source and appropriate joint design.
Other objects and advantages of the invention will be apparent from the following detailed description of illustrative embodiments, taken in conjunction with the accompanying drawings.